Published German patent specification DE-AS 20 05 457, discloses an exemplary IFF system which operates on the radar (particularly secondary radar) principle. In the answering unit only one of many possible interrogation codes is recognized as valid in a certain time range, whereupon an answer signal may be triggered. The answering unit stores not only the currently valid interrogation code, but also future and/or past codes. By comparing the received code with the stored codes, the answering unit may be synchronized with the interrogating unit.
German Patent 29 39 959 (which corresponds to U.S. Pat. No. 4,249,265) discloses an IFF system which operates with coded light signals. It includes an omnidirectional receiver and an optical system that selectively reflects the coded light signal in the direction of incidence.